1. Field of the Invention
The field of the present invention is manufacturing processes, particularly manufacturing processes which construct articles using layered tapes.
2. Background
Evolving industrial needs have created a growing dependence on lightweight material solutions and rapid machining processes. Composite materials are a leading force in this industry, providing extremely light weight parts that can be designed to suit specific thermal and structural needs. For example, the aircraft industry depends on composites for many different parts used in constructing passenger airplanes, to the extent that some of the newest models are constructed from between 50% to 60% composite materials. The growing dependence of many industries on high quality composite parts means that a reduction in production costs and an increase in production speed and quality are needed. The still-developing industry of composite parts manufacture, however, still employs relatively slow and expensive methods of production. In addition, the current methods are limited in the types of parts that may be produced.
One advance in the manufacture of composite articles involves the layering of composite tapes, or tows, across a mold at prescribed orientations. The tapes typically consist of parallel, unidirectional fibers within a resin matrix, and numerous layers of adjacently laid tows may be stacked in a series of plies to create a final article having a desired thickness. The resulting composite substrate has material properties given by the filament properties within the tows and the orientations of the various plies. While much of this manufacturing process can be automated, difficulties with automation arise as the shape of the manufactured articles become more complex. For geometrically complex articles, or even those with large surface gradients, the tows are often laid by hand or their positions are manually entered into the automated system (as opposed to the positions being calculated by the system itself). Thus, event with currently available automated systems, complex parts can be time consuming and expensive to manufacture.
Automated tow laying systems currently employed often use an open bay, gantry style machine to lay unidirectional fiber tows of a given width on a predefined surface mold. Such systems generally have three to six degrees of freedom and are computer numerically controlled. Many different processes of controlling these automated systems have been developed over time with the primary goals being (1) minimizing gaps between tows; (2) maximizing coverage by the tows; (3) minimizing strain placed on individual tows; and (4) minimizing tow wrinkling and puckering. In furtherance of these goals, so called “natural path” algorithms were developed to minimize strain, wrinkling, and puckering.
Although many different types of gantry style systems my be employed, a typical system with which a person of skill in the relevant arts might be familiar is described below. In such a typical gantry style system, the gantry head includes a robotic applicator that is suspended on parallel rails above the part being assembled. Linear movement of the head along the rails provides two degrees of freedom. Two additional degrees of translational freedom and three degrees of rotational freedom are be built into the gantry head or the robotic applicator itself. This arrangement permits the gantry to operate in an xyz defined space, meaning that any point along a surface mold are capable of being defined by a global Cartesian coordinate and a normal vector to establish the orientation of the point. Thus, motion of the gantry head and robotics applicator is directed by providing a series of incremental coordinates and their associated orientations. This sort of system has greatly simplified motion calculations and control for automated tape laying systems. Further, this simple input allows the gantries to be constructed to cover large working spaces, move at high velocities, and work with both high precision and accuracy.
The gantry head also includes a carrier and a roller, which are used to apply the tows to the mold, and a tow spool from which tow is supplied during application. The carrier and roller control tow alignment, orientation, and tension during placement. The gantry head might also include an optical device to aid in monitoring the tow laying process and the tow material itself, as flaws and weaknesses in the tow material are common due to difficulties in manufacturing consistent, uniform composite material. A heater within the head increases tow temperature during placement to heat tow to an appropriate bonding temperature, and a cutter separates a given tow from the tow spool. At the interface between the tow and the substrate, a spring loaded shoe maintains near constant pressure over the varying contoured surfaces of the substrate to control the trajectory of the applied tow and to minimize slippage off the intended tow path. The shoe is followed by a compaction roller to ensure uniform bonding over the entire tow width and to prevent air gaps. The applied pressure is typically dependent upon the type of tow material and resin being used. Sensors are also often included to detect stresses placed on the gantry head during tow application. Low stress serves as an indicator of poor bonding between the tow and the substrate, while excessive stress serves as an indicator of path definition errors, which can lead to tow wrinkling or tearing.
Such gantry systems may be employed with varying degrees of automation and human interaction. In “single phase” systems, the process is fully automated, from feeding, placement, and cutting of the tow. In “two phase” systems, tow placement is separated from tow cutting. Here, tows are cut to specified shapes and sizes in a pre-processing stage before being spooled and placed onto the gantry head for application. In “two phase” applications, since all cuts and paths are predefined, the application stage can proceed more quickly; unfortunately, the pre-processing stage can be very time consuming and costly. Finally, some gantry systems are designed as “dual phase” systems, with which either “single phase” or “two phase” processes may be employed.
The overall success and speed of almost any gantry system, including the quality of the articles produced, however, is highly dependent upon the definition of the tow paths. Simply put, poorly defined paths yield poor quality parts. Path generation, in it's simplest form, can be and was done by hand. But, hand methods are extremely slow, have very low repeatability, and provide no ability to analyze the product being produced prior to completion of production. Gantry systems have been employed to increase repeatability and speed, but gantry systems that rely on input from the operator for initial tow path definition still do not provide any pre-production analysis ability. In view of this problem, “natural path” tow path definition methods were developed. In the natural path process, tow paths are defined by the contours of the surfaces using the state of lowest stress induced in applied the tow strips. Natural path techniques further evolved so that tow paths were defined by natural geodesic paths.
More recently, natural geodesic path determination have been refined to vary the initial angle of the tow path to within a maximum threshold, thereby reducing application deformities and gap separations between adjacent tow strips. In this technique, each subsequent tow path is calculated from the previous path. However, this means that if a non-optimal first path is selected, high strains and wrinkle areas might be produced after many adjacent paths are laid.
In another advance of natural geodesic path determination, isoparametric projections and numerical convergence techniques have been employed to enhance tow path definition. This technique, however, appears to be overly sensitive to local extremes of the substrate and may result in convergence to tow paths that overlap undesirably.